When you say "natural without any lube" were you also not using a condom? Again, the condom issue makes a difference, possibly a huge one.
If you *WERE* using a condom, I'd say that's a pretty dead giveaway that you're dealing with a latex allergy.
If you weren't, see what the Diflucan does for you, because if it isn't a substance allergy, my guess is that you have some low-lying infection or irritation that the friction of sexual activity is irritating. The Diflucan may get rid of it.
In the meantime, you may want to have your BF get tested for all STDs as well as to get him tested for fungal or bacterial infections of the skin in and around his genitals. It is not impossible for men to harbor infections in the skin down there, not have symptoms, and transmit them to the women they have sex with. No fun!
The reason to get him checked out is that only one of you is on Diflucan. even if the Diflucan clears out an infection on your end, he may still be infected with whatever it is and would be able to pass it right back to you as soon as you were done with the Diflucan.
And we're glad we can help. Just remember that we're not actually doctors, so we can't give you definitive medical information, just suggestions.
(p.s. You can respond to existing threads here. It's only in Ask A Sexpert that you can't reply to threads.)
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